The parser basically works in a two phase process, first it scans using a very fast non-backtracking regex, to see if it can find any opening tags it recognises. If it succeeds it marks the tag with a control character, then runs a looped slower regex which scans for a complete open and close tag set. When it has something it knows is valid, and does not contain a nested tag set (as determined by negating the control character), it then executes the relavent code and substitutes the return value into the document. The loop continues until there are no more matching sets to process.
The first phase negates the backslash to prevent it from picking up on and marking out the close tags so the tag you mentioned will probably be ignored by both phases and remain untouched in the final output. I'd have to check back on the actual regex used to be certain but I'm pretty sure that is correct.